The present invention relates to electron guns having arc suppression resistors and particularly to an electron gun for a cathode ray tube, such as a color picture tube, in which the arc suppression resistors are located in a zero electric field gradient area of the electron gun.
A conventional color television picture tube consists of an evacuated envelope having a neck portion, a faceplate, and a funnel portion therebetween. An electron gun is disposed in the neck portion of the envelope, and a tricolor emitting phosphor screen is disposed on the interior surface of the faceplate. A shadow mask is located between the electron gun and the screen, in spaced relation to the screen. The electron gun comprises a plurality of electrodes for focusing and accelerating three electron beams toward the phosphor screen. The electrodes are supported as a unit from at least two elongated, axially-oriented support rods in the form of glass beads. The beads have extended surfaces closely spaced from the facing the inner surface of the glass neck. Typically, several high voltage and low voltage electrodes are serially attached to the support rods along the electron beam paths to facilitate the focusing and accelerating of the electron beams. The high voltage electrodes typically operate at an ultor potential of about 30 kilovolts, and the low voltage electrodes typically operate at about 8 to 10 kilovolts or less; however, in some electron guns, an intermediate potential of about 12 kilovolts and a low potential of about 8 kilovolts or less are utilized. A conductive coating having a resistance of about 100 ohms is disposed on the interior surface of the funnel portion of the envelope. The interior conductive coating operates at ultor potential. Bulb spacers mounted on the electron gun electrode nearest the phosphor screen contact the interior conductive coating to provide ultor potential to the electron gun. An exterior conductive coating, electrically isolated from the interior conductive coating, is provided on the outside of the funnel to facilitate grounding of the envelope. The interior and exterior conductive coatings on the funnel serve as a large capacitor which filters the high voltage.
The large voltage difference established between the high voltage and low voltage electrodes in the electron gun creates a possibility of arcing between the electrodes. The possibility of arcing is increased by irregular electrode surfaces, foreign matter in the interelectrode gaps and by misalignment or improper spacing between electrodes. When an arc occurs, the high voltage filter capacitor will, within a few microseconds or less, discharge its stored charge.
Because the instantaneous peak arc currents can approach hundreds or even thousands of amperes in magnitude, great destruction can be caused by such arcs. The external electron gun circuitry can be damaged by transient currents and voltages induced into the associated receiver circuitry. The gun electrodes can be burned or eroded to the point of inoperability, and electrode material may be sputtered onto adjacent surfaces resulting in the creation of leakage paths between tube elements.
In order to reduce tube arcing and to minimize the damage caused thereby, it is common to design cathode-ray tubes with maximum electrode spacings, to minimize field gradients and to incorporate arc suppression systems into the tube.
Such an arc suppression structure is disclosed in U.S. Pat. No. 4,345,185 issued to Y. Kobori on Aug. 17, 1982 and discussed by Y. Kobori et al. in their paper entitled, "A Novel Arc-Suppression Technique For Cathode Ray Tubes", presented at the IEEE Chicago Spring Conference on Consumer Electronics, June 19, 1980. The structure disclosed in the Kobori patent requires that a ceramic resistor be connected between the G3 and G5 high voltage electrodes (typically 30 kV) and that another resistor be connected between the low voltage G4 electrode and the stem lead attached thereto. When arcing takes place between adjacent electrodes, the arc current may flow either from the G5 through G3 to the G2 or to the G4. A problem occurs when the initial arc and the resulting plasma generated thereby results in additional arcs, e.g., cascading arcs, between the other electrodes of the electron gun. A cascading arc is herein defined as a succession of rapidly initiating arcs between electrodes in high field regions of the electron gun which permit a sufficiently high arc current to pass between electrodes of the electron gun and into the receiver subsequently causing damage to the electron gun components and to the associated gun circuitry.
Therefore, an arc-suppression system must be able to protect the electron gun not only from the effects of individual arcs but from the effects of cascading arcs. Arc suppression systems that protects electron guns from individual arcs and greatly reduces the probability of the occurrence of cascading arcs and the damage therefrom are described in U.S. patent application Ser. Nos. 424,136, and 424,140, filed Sept. 27, 1982 by R. Stone and assigned to the assignee of the present invention. The Stone patent applications are incorporated by reference herein for the purpose of disclosure. The Kobori patent referenced above and the Stone patent applications utilize resistors attached to the surfaces of the electrode members to provide arc suppression. The resistors are located between the external surfaces of the electrodes and the inside surface of the neck of the envelope so that the resistors disrupt the electric field in the vicinity of the electron gun. In some embodiments of the first named Stone patent application and in the Kobori patent, the arc suppression resistor spans an intermediate electrode which is operating at a significantly different voltage from that applied to the electrodes to which the resistor is attached, further disturbing the electric field of the gun. Furthermore, unless the resistor is attached carefully to the electrodes of the electron gun, burrs or braze material at the points of attachment may provide a site on the electrodes for the initiation of an arc. Thus, it is desirable wherever possible, to locate the arc suppression resistor where it will not disturb the electric field of the electron gun.